In recent years, antenna devices using linear elements have been investigated for the purpose of use in wireless communication, such as in a keyless operation system for an automobile. Conventionally, a monopole antenna having a length ¼ that of the working wavelength of the antenna with respect to the ground plane and in which a wire element is disposed is generally used as an antenna device that uses linear elements. However, because this monopole antenna is large and tall overall, inverted-L antennas have been developed in which this monopole antenna is folded at an intermediate point to reduce the size and height.
Furthermore, in this inverted-L antenna, matching a 50Ω power feed line is very difficult because the reactance, which is determined by the length of the horizontal portion of the antenna element that is parallel to the ground plane, is the capacitance, and is a large value. Thus, conventionally, in order to facilitate matching between an antenna element and a 50Ω power feed line, what is referred to as an inverted-F antenna has been proposed. This inverted-F antenna is one in which a stub is provided that connects the ground plane and the radiation element near the power feed point that is provided at an intermediate location on the antenna element. Thereby, the capacitance due to the reactance is neutralized, and matching a 50Ω power feed line is facilitated. For example, in Japanese Laid-Open Patent Application No. 2006-197528, an inverted-F antenna has been proposed that is applied to a folding portable wireless device, and provides an antenna element that is disposed on a printed wiring substrate and is folded perpendicular to a flexible flat cable that is connected to the printed wiring substrate. In this inverted-F antenna, the antenna element is folded in a vertical direction with respect to the printed wiring substrate.
However, in the above conventional technology as well, the following problems remain. Specifically, in the conventional antenna, in the case in which the various arrangement conditions are to be changed, the polarization (vertical and horizontal polarization) is designed with respect to the principal polarization as required by the use conditions, and due to being dependent thereon, handling polarization improvements using the same antenna profile is difficult. Thus, methods in which design changes are made to the antenna profile in order to improve polarization have been considered, but there are limits to design changes due to dependency on the size of the case and there are frequently difficulties in terms of the cost of producing metal molds and the like. Thus, a method in which a metal plate is disposed to change the polarization forcibly has been considered, but there the drawbacks that directionality is limited, antenna characteristics deteriorate, and the polarization cannot be easily improved.
In addition, in the case of the technology disclosed in Japanese Laid-Open Patent Application No. 2006-197528, the characteristics are improved by folding the element in a length or width direction with respect to a flexible cable, but because the improvements in characteristics are dependent on the surrounding environment, the polarization and directionality cannot be improved, and downsizing and increasing the thinness are difficult. From the above point of view, in the case in which the arrangement conditions are changed in the conventional technology, in the same antenna, substrate, and case, there are the drawbacks that improving the polarization, obtaining high gain, downsizing, and increasing thinness are difficult.